This project examines the neural circuits involved in the appreciation of oral-facial sensations, particularly the circuits mediating pain and temperature sensations. Incoming sensory information may be modified by neuronal activity in many areas of the nervous system before being relayed to higher brain centers. Neuron marking techniques, in which a foreign protein (horseradish peroxidase) is picked up by axonal endings and carried back to their neuronal cell bodies, accurately demonstrate which neurons participate in a given neural circuit. Previous experiments in this study identified the neurons that relay incoming oral-facial sensations to higher brain centers. Experiments conducted during the past year have identified the neurons that can modify incoming sensory information and that may, in part, mediate analgesic and hyperalgesic states.